Television systems typically use a pair of interlaced fields to compose an entire frame or picture on the display. According to the National Television Systems Committee (NTSC) standards, in television system in the United States, 29.97 frames are transmitted per second as 59.94 fields, so that the display is updated 59.94 times per second, which is fast enough to avoid the appearance of the display flickering. In such a system, one field will contain the odd numbered scan lines of the image, while the other field contains the even numbered scan lines.
When the television program is generated from an interlaced source, such as a video camera, these fields represent different moments in time. For example, an actor walking across the set may be in a different location from one field to the next. Also, camera motion may cause the entire image to shift between fields. On the other hand, if the program is generated from a progressive (non-interlaced) source, such as film, the fields represent the same moment in time.
Video on Demand (VOD) servers that generate dynamic trick mode sequences typically use the intra-coded pictures present in the stream to generate these sequences. As understood in the art, a “trick mode” is a mode other than normal playback, such as fast forward, rewind and so on. A trick mode sequence typically consists of the intra-coded picture followed a series of inter-coded pictures (P-repeats) that force the decoder to redisplay the current image. Traditional server architectures that rely on trick mode files will typically re-encode the intra-coded pictures and are thus able to groom them to their own requirements.
Content that is encoded for VOD use is typically encoded as a stream of frame pictures, where each frame contains the contents of two fields. However, it is generally not possible to separate the coded frame into its fields without decoding the frame. In a trick mode sequence, both fields are displayed for the original intra-coded frame, and the subsequent repeat frames cause both fields to be redisplayed.
Hence, if the original content originated from a progressive source, such as movie film, the two fields represent the same instant in time and the effect of redisplaying the two fields is a stable image. However, if the content originated from an interlaced source, the images in the two fields may be different and the visual effect will be that of redisplaying two alternate, superimposed images, each at 29.27 or 25 times per second, which results in flicker in the picture that can be very distracting.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.